Quiet Professionals, Noisy Machinery

Testing Polymer Receivers to Destruction: Factory and Printed

Here’s another embedded video from Full30.com’s InRange TV, where Ian and Karl do their level best to destroy a Cav Arms polymer lower.

They step on it, stomp on it, run it over with a Jeep, and shoot holes in it, and still it keeps on shooting. One is reminded of the old Timex ads, “Takes a licking and keeps on ticking.” Maybe it should be “Takes a drilling and it keeps on killing (IPSC targets).”

We’re not really shocked by this. We had AKs and SKSes in the foreign weapons arms room in 10th Group that were Vietnam captures, complete with bullet and claymore holes, and they all worked. (We kind of doubt their previous owner Mr Nguyen was still in such adequate operating condition). And we’ve seen ARs take some pretty brutal treatment and keep on shooting, including carbines that would still chamber rounds after their plastic was all burned off and their magazines blown out by a helicopter post-crash fire (we didn’t shoot them, though), and an M16A1 that still functioned (albeit inaccurately) with the barrel bent 30º off axis at the FSB1 (it was under a trooper’s armpit when he executed a really craptacular PLF2, dislocating his arm and bending the rifle).

A really good design is overwrought enough that it can be degraded by wear, corrosion, or, yes, combat, a good bit before it fails to function. And a really outstanding design delivers that with the smallest weight and bulk penalty possible.

Cav Arms made quite a few of these lowers out of durable Nylon 6 before the company was singled out for destruction by the ATF, which is a long story and off this topic. (A seemingly complete technical history of the Cav Arms lower has been prepared by Russel Phagan, aka Sinistral Rifleman, who assisted in the video). A successor manufactures the lowers today. (But the most significant thing about the lower wasn’t the company’s grim fate; it was that the lower was redesigned from the ground up to be made of polymer, to take advantage of this material’s strengths, and to shore up its weaknesses).

As Ian points out towards the end of the video, a polymer lower designed to be a polymer lower is a better bet than one that is just a molding of the traditional 7075 alloy machined forging. (Conversely, a steel receiver that follows the form factor of the alloy lower is going to be overstrength and overweight). These follow from the differences in the strengths of the three materials.

Ian notes the weakness of the buffer tower if the normal lower receiver is modeled in anything other than metal, and that gibes with the results that early lower-receiver 3D printers had, substituting much weaker ABS or PLA material for the 7075. The first point of failure to be made manifest was the buffer tower area. This led to reinforced buffer towers and ultimately such heavily-reinforced lower-receiver designs as the modern Aliamanu-Phobos.

Along with the reinforcements named in that slide, the massively reinforced buffer tower is evident. But even this beefy design can fail. This one started to delaminate with just 20 rounds fired. Test firing the lower:

Here’s the first image of the delamination. Since all the fire control group parts are above the delamination line, the weapon should still operate, but this obviously bodes ill for any probability of it surviving further testing. (Yes, these do embiggen for more of a close-up look).

Here’s the other side at that 20-round point:

Firing more rounds just cause more failure, in this case it seems that the area around the grip screw also began to delaminate, releasing the grip:

At this point, stick a fork in it, it’s done.

Others have had much better results, including from pretty low end perimeters, and the equipment and parameters that FOSSCAD member trouble1 used didn’t seem out of step with what the successful printers did. But you can’t call this a successful print. It seems highly probable that there is some failure in the print setup or materials (moisture in the filament?) that no one has figured out yet.

That delamination is an interesting failure mode that’s fairly common in fused filament fabrication printing, is only one reason the technology is not yet ready to compete head-to-head with plastic injection molding. The much slower production of the additive process, and its higher per-unit variable cost, also argue against this for production. However, injection molding, with its generally higher fixed costs (for tooling), is unsuitable for prototyping and very short production runs. A hybrid of technologies that uses printed molds to reduce that fixed cost for short runs offers the potential of closing the gap. But a proper part is a part that is designed in conjunction with its manufacturing technology — engineered for production from Day One, with materials chosen to meet the mission and simplify, speed up, and save money on production.

As Ian noted about the Cav Arms polymer lower (which is injection molded), it’s necessary to design the part to make best use of the materials and technology. Simply trying to reverse-engineer a popular firearm in a new material or manufacturing approach will only take you so far. It may, given enough iterations, be far enough.

Notes

FSB = Front Sight Base, the triangular-shaped forging that holds up the front sight on the nose of AR-15 series rifles through the early M4A1. It also locates the gas tube and hosts the bayonet lug — a busy small part.

PLF = Parachute Landing Fall, a specific roll that reduces the risk of injury when a para touches down.

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Wow, that was impressive. Keep this up and I’ll want one bad enough to emigrate. You’ve sold me on the AR, finally, even undoing the damage done by an A1/203 that was merely a collection of scrap existing in somewhat close proximity.

This thing has been getting whacked good at the grip to lower join. Hold harder at a front grip? Redesign so’s there’s no impact there?Put some serious amt of plastic there?
A you tube vid on a wasr 10 tuning showed how to round the back of the hammer for smooth bolt travel, made the gun work easier. Much easier. Somehow the ar has to be smoothed out to lower the impact on the lower. impact over a longer time might help. I know nothing. Norinco ak guy.
Keep trying guys.

We’ve been experimenting with acetone vapor on our ABS prints . While most folks do this because of the smooth finish it provides, it also bonds the layers much more tightly together, which prevents that kind of delamination (on smaller prints at least) . Some makers have put together complex heated vapor baths, but we’ve had good results with a splash of acetone simply evaporating off a wad of paper towels in a glass or metal container. Then again, our shop is pretty warm in the summer.

trouble1
Apr 27: @ArmaDelite bed 90 degrees, acetone treatment, printer is in an enclosure? So this is not a normal phenom?

DrafterAnon
Apr 27: @ArmaDelite An irregularity in the filament, perhaps?

So there was a failure despite an attempt at acetone (presumably vapor) treatment. If he ever followed up I didn’t see it, he went on to print the Bolt model AR lower.

I wonder if he had moisture in the filament. What do you think?

Thanks for the links and comment.

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WeaponsMan is a blog about weapons. Primarily ground combat weapons, primarily small arms and man-portable crew-served weapons. The site owner is a former Special Forces weapons man (MOS 18B, before the 18 series, 11B with Skill Qualification Indicator of S), and you can expect any guest columnists to be similarly qualified.

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